This was likely done to balance out the power of moves and make it so that physical attackers are no better or worse than special attackers. For example, physical attackers get access to the elemental punches with base 75 power while special attackers get the elemental beams with base 95 power. It is possible that Game Freak wanted to balance this a bit without disappointing fans too much with a 20 base power drop. It happened to Hidden Power, Draco Meteor, Hydro Pump, Leaf Storm, the list goes on. On the other side physical moves' base powers increased, such as Crabhammer, Knock Off, Skull Bash, etc. which makes this theory seem more convincing.
Game Freak may also increase the power of moves they want to see used more. An example of this is Power Gem which went from 70 to 80, likely due to the lack of good special Rock moves. It may also have been done to compliment a new Pokemon. Mega Heracross got Skill Link and was granted access to Pin Missile, which experienced an increase of 11 base power (14 to 25) as if to make people more inclined to use Skill Link to its fullest.
Game Freak have changed moves every generation, so it may have even been a trend that they wanted to continue.
